The weaknesses here are that it gets pretty wordy in the body and you're unlikely to be able to get it down to listing a color only once. But you might be able to do it only two or three times and you can group those colors together, perhaps in their own sheet. Now when you want to change the color scheme they're all together and the change is pretty simple.

But, yeah, my biggest complain with CSS is the inability to define your own constants.